About New Kinds of Telescopes, Especially for Handheld Use.
Ueber neue Arten von Fernrohren, insbesondere fuer den Hangebrauch.
Lecture held at the session of the
Society for Advancement of Industrial Activity on January 7, 1895
by Dr. S. Czapski,
Scientific Colleague of the Optical Laboratory of Carl Zeiss in Jena
Continuation, Number 5; from the 'Central-Zeitung fuer Optik und Mechanik'
(Journal for Optics and Mechanics), Berlin, 01 Mar., 1896
Translation by Ilse Roberts and Peter Abrahams
The purpose of such a field glass is twofold. It allows the observer
to remain behind cover, viewing ‘around a corner’ with only the objective
unprotected and visible to the object or vulnerable to a bullet. If the
offset of the objective from the ocular is sufficiently large, the head
and body of the observer can be completly hidden without limiting the
observation. For simple telescopes, this is the only advantage, but it
alone should make these instruments useful for military purposes, and
there are many other circumstances in which they could be useful.
Fig. 16 shows a simple telescope of this type in three views. The
sequence of prisms, derived from the first principle of the prism system
(fig. 6), is similar to fig. 13. There are 4 prisms, P is an isosceles
right angle reflecting prism, just behind the objective O. The remaining
three prisms are shown as a larger reflecting prism q with two reflecting
surfaces, and another smaller prism p attached to it. They are installed
in the housing G that holds the ocular o of the telescope.
The second purpose of such a telescope is more important, and concerns
its function when joined with another for binocular use. Such binoculars
are desirable for esthetics alone, especially for instruments with
erecting prisms, which have an asymmetrical appearance as monoculars, with
an accentuated offset between the objective axis and the ocular axis.
There have been attempts to cover up this asymmetry with a symmetrical
housing that does not resemble its interior, but in our opinion these have
been less attractive than a tightly fitting housing. If two of these
asymmetrical bodies are connected in a symmetric way, the transformed
instrument does not offend the eye, and even gives the opportunity for
pleasing forms of execution.
By connecting two telescopes into a binocular, the opportunity arises
to increase or decrease the distance between the objectives without adding
any additional optical elements to the system. The distance can be larger
or smaller than the distance between the oculars, the minimum being
determined by the dimensions of the prisms, or it can be enlarged almost
any amount. To accomplish this, the user need only adjust the connection
between the telescopes [separate the objectives by opening the hinge
between the oculars]. If this connection is designed so that when the
oculars (oo in fig. 17) are adjusted to the user’s interpupillary
distance, the tubes form an acute angle towards the objectives, as in 17a,
the objectives become closer to each other than the oculars are, until
they touch. With small objectives, this can give nearly completely
unplastic, 2 dimensional images, although this is not usually
advantageous. In fig. 17, the axes of the telescope tubes EE contain both
the ocular axis and the objective axis. If the connection between the
telescopes is designed so that the axes EE diverge to the outside, when
adjusted to interpupillary distance (fig. 17b), the distance between the
objectives becomes larger than the interpupillary distance. This can give
the user an enhanced perception of depth or space, an increased plasticity
of the images in comparison to a binocular of similar specifications with
objectives spaced the same distance as the oculars. This increase can be
measured in comparison to a common binocular, and is exactly equal to the
ratio of the objective distance to the ocular distance. The increased
plasticity in comparison to a naked eye view equals the linear
magnification times the increase in objective distance.
This phenomenon, and its qualitative and quantitative relations, is
not a new discovery. Helmholz, the recently deceased leader in physics
and physiology, recognized all this long ago, pursued all the theoretical
and practical consequences, and described it in his famously clear and
precise manner. Any explanation here would be only a repetition of his
research in Physiologic Optics (1st edition, especially pp. 647 and 681),
and in another paper (Pogg. Ann. Bd. 102, pp.16a--175, 1857). These
papers are recommended to anyone interested, for they are an opportunity
for instruction and much enjoyment, to those who read them.
Helmholtz called the instrument which he constructed the
“Telestereoscope”. We prefer “Relieffernrohr” [relief telescope], for
common use. These use reflections of the image by arranging the (already
required) optical elements of the telescope along its longitudinal axis.
The Helmholtz invention was little used in the years since its
publication, which astonishes anyone who has experienced the charm of
these instruments. I believe that inadequate execution, especially the
adjustment of the axes of each telescope by the mechanics and opticians
entrusted with it, is to blame. If this is so, that these instruments
cannot provide fusable images to the eyes, then the eyes are forced to
adjust to the optics, which soon causes disgust with such instruments.
However, with correct adjustment, the highly plastic images produced by a
Telestereoscope of any variety, give a special charm to the landscape from
the increased relief they provide.
It is often observed, that a first time user of these instruments is
unaware of their pecularity and hardly feels their charm. With increased
use of these optics, an increase in receptive feeling for their images
occurs, which increases the enjoyment and also the capacity for (or the
consciousness of) plastic seeing, even with the naked eye. Perhaps the
capacity to see stereoscopically has atrophied for many people from lack
of exercise, so that it is slowly strengthened and awakened by practice.
Perhaps the sense of depth, like the sense of color, varies with the
population and with attention and practice. If this is so, the present
era could be the entry to an age of better development of that sense. At
the least, the inclination to use the stereoscope for photographic images,
and the efforts to give these instruments a functional design, is quite
noticeable to those who follow photographical literature. The binocular
telescope here described will make its contribution by granting an
increased enjoyment of viewing colorful, moving, and real nature; like the
stereoscope gives for photographs.
(*Esthetic enjoyment requires less magnification (larger f.o.v.) and
increased distance between objectives. Increased plasticity does not give
distant objects an appearance of greater magnification, but the observer
seems to see smaller plastic details in them, as described in Helmholtz.)
This ‘sense of depth’ is as worthy of attention and development as any.
Nature has given us equipment to view our world with two eyes, from two
different viewpoints at one time. The ability to unify these two distinct
images and to infer the spatial arrangement of the objects in the images,
is a source of much esthetic pleasure and also a valuable skill for
orientation. The differences between the two images from the two eyes are
seen as characteristics of the various distances of the objects in view,
and are seen with a sensitivity of perception that is unrivalled.
Binoculars with increased distance between the objectives can heighten
this sense, as compared to binoculars of the same power, and of normal
configuration. This is obvious to any attentive observer using these
instruments. Landscapes, that appear as a uniformly extending plane
through a telescope or even a common binocular; when observed through a
relief telescope reveal at first glance diverse stratifications, waves,
slopes, precipices, and cracks. The user can almost see the air which
separates the features of the terrain, that are projected onto each other
in a simple telescope. The study of a terrain according to its depth
formation is possible, adding to the information on width and height seen
with common telescopes.
To obtain these advantages, it is necessary to adjust these telescopes
to provide images that are magnified to exactly the same size, with
exactly parallel optical axes with no divergence. An adjustment to
individual interpupillary distances is required of the connection between
the telescopes, and a focusing mechanism for each side is needed. The
degree of plasitcity of the fused image depends to a great degree upon the
resolution with which the two individual images are seen. Since one eye
of a person often has small differences from the other eye, the focusing
requirement is indispensible. These requirements have been met in the
Zeiss instruments.
Other than meeting the required stability, the type of mechanical
connection between the telescopes is immaterial. They can be joined by a
hinge or another joint which permits adjustment to interpupillary distance
without losing parallelism of the optical axes. They can also be set in a
rigid connection, without adjustment for interocular distance; or with
other forms of adjustment. Required is a mechanical connection that
maintains the large distance between the objectives when interocular
distance is adjusted.
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